A liquid crystal display of the active matrix type has a character display portion in which a number of pixels are arranged in rows and columns. Each pixel is disposed in a region surrounded by two neighboring scanning signal lines, or gate signal lines, and two video signal lines, or drain signal lines, which intersect the scanning lines. A plurality of scanning signal lines extend horizontally to form rows. A plurality of video signal lines extend vertically and cross the scanning signal lines to form columns.
The aforementioned pixel consists essentially of a liquid crystal, a transparent pixel electrode, a common transparent electrode disposed on the opposite side of the pixel electrode from the liquid crystal, and a thin-film transistor. Individual pixels have their respective transparent pixel electrodes and their respective thin-film transistors. The pixel electrode of each pixel is connected with one electrode (hereinafter referred to as the source for convenience) of the thin-film transistor. The other electrode (referred to as the drain) of the thin-film transistor is connected with one of the video signal lines. Gates are connected with the scanning signal lines.
Liquid crystal television displays of the active matrix type which have been recently commercialized require a number of steps to fabricate thin-film transistors on a glass plate. This means that 7 to 12 photographic masks are needed. This series of manufacturing steps accounts for a large proportion of the whole manufacturing cost. Different masking levels are often subjected to different photographic processes and, therefore, the rate of opening, i.e., brightness, is frequently sacrificed to give allowance to mask alignment.
In the active matrix type, DC component tends to be applied to the liquid crystal because of electrostatic coupling between scanning signals. Reduction in the DC component will contribute to increase in the life of the liquid crystal and also prevent nonuniformity of displayed image.
Usually, unwanted incident light to the front surface of a liquid crystal display is blocked by a film formed on thin-film transistors, while unwanted back light is intercepted by opaque gates. The present inventors et al. made various experiments and have found that gates of normal size cannot sufficiently block light. This tendency becomes especially conspicuous in projector-type displays using strong light sources.
When light falls on the amorphous semiconductor layer of a thin-film transistor, pairs of electrons and positive holes are created. This deteriorates the cutoff characteristics of the transistor. For this reason, the device is required to be designed so that the amount of light hitting this layer is reduced to a minimum. To make characters visible, natural light or artificial light emitted by an indoor electric lamp is caused to enter the device from the front surface of the device, or light emitted from a fluorescent lamp or the like is made to enter the device from the rear surface of the device.
As the above-described liquid crystal displays are fabricated in larger sizes, the sizes of the pixels tend to increase. As an example, the dimensions of pixels in the character display portions of liquid crystal displays fabricated in the past were 0.2 mm.times.0.2 mm. The present inventors have developed a liquid crystal display having pixels measuring 0.32 mm.times.0.32 mm.
During manufacture of liquid crystal displays of this kind, foreign matter such as dust may be introduced into the character display portion. Also, foreign matter may adhere to masks used in photolithography. If foreign matter is introduced between the source (or transparent pixel electrode) of a thin-film transistor and the drain or exists between them, a short circuit occurs between them. Then the short-circuited pixel malfunctions, producing a point defect. If similar foreign matter is introduced between the source (or a transparent pixel electrode) of a thin-film transistor and the gate or exists between them, a point defect is formed in the same way. Therefore, the present inventors have discovered the fact that as the size of each pixel increases, such point defects or loss of pixels in the liquid crystal display becomes more conspicuous.
A device in which the size of gate electrodes is made larger than the semiconductor layer is known as disclosed in Japanese Patent Laid-Open No. 17,962/1985. However, simply increasing the size of gate electrodes will increase the parasitic capacitance between gate and source, which in turn increases the DC component to an intolerable level, the DC component being applied to the liquid crystal in response to a scanning signal. In total this demerit is too large. Hence, it is difficult to put the device into practical use.